Organic electroluminescent devices are self-emission type display devices and are expected for use in displays and lights. Organic electroluminescent displays have advantageous display performances such as higher visibility than conventional CRTs and LCDs, and no viewing angle dependency. Organic electroluminescent displays also have an advantage that they can be made lighter and thinner. Meanwhile, organic electroluminescent lights can be advantageously lighter and thinner and also, using a flexible substrate, organic electroluminescent lights may have a shape conventional lights cannot have.
An organic electroluminescent layer of an organic electroluminescent device has a multi-layered structure containing a light-emitting layer, a transparent electrode and other layers. Therefore, when light is emitted at an angle equal to or higher than a critical angle determined based on the refractive index of the organic electroluminescent layer and the refractive index of a medium into which the light is to be emitted, the light cannot be emitted to the air, totally reflected, and confined in the interior of the organic electroluminescent layer, so that the light is lost. According to calculation based on the classical Snell's law of refraction, when the refractive index n of the organic electroluminescent layer is 1.8 (the refractive index n of the organic electroluminescent layer is 1.7 to 1.85 according to NPL 1) and the distribution of light emitted from the organic electroluminescent layer is a Lambertian distribution, the light extraction efficiency of the light emitted to the air is only about 30% due to the difference between the refractive index of the organic electroluminescent layer and the refractive index of the air. The remaining light accounting for about 70% cannot be emitted to the air since it is confined in the interior of the organic electroluminescent layer due to this difference in refractive index.
Solving the above problems to improve the light extraction efficiency requires preventing reflection occurring between the organic electroluminescent layer and the air due to the difference between their refractive indices, in particular, preventing the occurrence of total reflection. Provision of a fine particle layer serving as a light extraction layer is one effective method for preventing the occurrence of total reflection, and there have been various proposals of providing the fine particle layer for changing the angle of light emitted from the organic electroluminescent layer.
For example, PTL 1 proposes an organic electroluminescence element containing a high-refractive-index substrate and a fine particle layer on a surface from which light is to be extracted. PTL 1 uses a reflective electrode of MgAg. In this proposal, however, light emitted from an organic electroluminescent layer is scattered by the fine particle layer, and some components are emitted to the air and other components are reflected into the organic electroluminescent layer as a result of back scattering. Extraction of the back-scattered components depends on the reflectivity of a reflective electrode, and the reflective electrode of MgAg is inferior to a reflective electrode of Ag in terms of reflectivity, which is problematic. In addition, since the light emitted from the organic electroluminescent layer travels at all angles, when the light travels in the thickness direction and reflected, scattered and totally reflected, the light hits the wall surfaces of the organic electroluminescent layer and is lost, which is also problematic.
Also, PTL 2 describes that the refractive index of a polymer in a fine particle layer is 0.9 times or more the refractive index of an organic electroluminescent layer. However, in this proposal, the refractive index of the polymer in the fine particle layer is 0.9 times the refractive index of the organic electroluminescent layer. When the distribution of light emitted from the organic electroluminescent layer is a Lambertian distribution, about 15% of the light cannot enter the fine particle layer. Thus, this proposal is unsatisfactory for making improvement in light extraction efficiency.
Moreover, PTL 3 proposes an organic EL device where a light-scattering portion is formed of opaque particles or transparent substances arranged on one plane thereof in a state of being dispersed or aggregated. This proposal, however, does not clearly describe the refractive index of the polymer in the fine particle layer, and when it is lower than the refractive index of the organic electroluminescent layer, total reflection occurs so that the light emitted from the organic electroluminescent layer cannot enter the fine particle layer. Thus, this proposal is unsatisfactory for making improvement in light extraction efficiency.